Interpretive Summary: Lowering the dietary crude protein level and supplementing with certain crystalline amino acids is an approach to reduce nitrogen excretion in pigs. The present study confirms that growth performance and lean tissue gain are similar between growing pigs fed a 16% crude protein diet or a 12% crude protein diet supplemented with adequate crystalline lysine, threonine, and tryptophan. Compared with feeding the 16% CP diet, offering pigs the amino acid supplemented, low-protein diet would result in less nitrogen excretion as reflected in the reduced kidney weights and concentration of serum urea plus ammonia nitrogen. Feeding pigs the amino acid supplemented, low-protein diet would reduce total heat production in growing pigs maintained under thermal neutrality or heat-stressed environment. The resulting energetic benefit, however, is insufficient to impact the metabolism of lean and fat tissues and to alter the rates of lean and lipid accretion in growing pigs.

Technical Abstract:
The study was conducted to determine the effects of feeding a 16% CP diet, a 12% CP diet, or a 12% CP diet supplemented with crystalline Lys, Trp, and Thr (12% CP + AA diet) in a thermal-neutral (23C) or heat-stressed (33C) environment on various body and metabolic parameters in growing pigs. No diet × temperature interaction was observed for any parameter (P >/_ 0.11) except for pigs' activity and pancreas weight. At 33C, pig activity and pancreas weight did not differ among dietary treatments (P > 0.05). But at 23C, pigs fed the 12% CP diet had greater activity than those fed the 16% CP diet or the 12% CP + AA diet (P < 0.05), while the pancreas weight was greater for pigs fed the 12% CP + AA diet than those fed the 12% CP diet (P < 0.05). Compared with 23C, 33C environment reduced activity, heat production, daily gain and daily feed intake, and affected the concentration and accretion of empty body water, protein, and ash, as well as weights of heart, pancreas, stomach, large intestine, and cecum in pigs (P < 0.05). Pigs fed the 12% CP+ AA diet attained similar levels of performance and rates of empty body water, protein, lipid, and ash deposition as pigs fed the 16% CP diet (P >/_ 0.08). Pigs fed the 12% CP + AA diet also had lower serum urea plus ammonia nitrogen levels (P < 0.01) and heat production (P < 0.05) compared with those fed the 16% CP diet or the 12% CP diet. These results confirm that growing pigs fed a 12% CP diet will perform similar to pigs fed a 16% CP diet if crystalline AA are supplemented to the 12% CP diet. The data further indicate that lowering dietary CP and supplementing crystalline AA will reduce heat production in growing pigs whether they are housed at thermal-neutral or heat-stressed environment.